One-way valves of the kind defined in the introduction are previously known. The Fres-co System from the Goglio Group and the Wicovalve® from Wipf are examples of valves of this kind that comprise a button-shaped cylindrical membrane retainer and a membrane. The membrane is the active part of this type of valves and it opens or closes the valve depending on the pressure relationship between the two sides of the membrane. The membrane is oiled to ensure that the valve is sealed.
This type of valve is used for coffee packages and packages intended for certain foods. In order to increase the shelf-life of these foods it is important that they be vacuum-packed, i.e. that as much oxygen as possible is evacuated from the packages. In some cases foods are packaged that are also cooked in the same package. During the cooking considerable amounts of water vapour are generated and the excess pressure expels the water vapour. In cooking by means of microwaves, the generation of water vapour is very intense and the flow through the valve is comparatively large. When the above described valves of conventional type are used, the oil is blown off by the water vapour, in many cases resulting in considerable impairment of the sealing capacity of the valve. At best, the valves are still sealed but they have instead become single-use valves. In some cases, it is desirable that foods that are cooked in conjunction with being packaged may be re-heated while still in the same package. In order to avoid having to pierce extra holes in the package it is essential that the valve is serviceable again when the package is re-heated, so as to prevent the package from bursting.
Another problem is the costs. Although these valves are comparatively simple structurally, they still constitute a considerable part of the total costs of the package.